selftests/kcmp: Always try to build the test

Don't prevent the test building on non-x86. Just try and build it and
let the chips fall where they may.

Add support for CROSS_COMPILE while we're at it. Also we don't need a
custom rule for building kcmp_test.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/kcmp/Makefile

-uname_M := $(shell uname -m 2>/dev/null || echo not)
-ARCH ?= $(shell echo $(uname_M) | sed -e s/i.86/i386/)
-ifeq ($(ARCH),i386)
-        ARCH := x86
-	CFLAGS := -DCONFIG_X86_32 -D__i386__
-endif
-ifeq ($(ARCH),x86_64)
-	ARCH := x86
-	CFLAGS := -DCONFIG_X86_64 -D__x86_64__
-endif
+CC := $(CROSS_COMPILE)$(CC)
 CFLAGS += -I../../../../usr/include/
 
-all:
-ifeq ($(ARCH),x86)
-	gcc $(CFLAGS) kcmp_test.c -o kcmp_test
-else
-	echo "Not an x86 target, can't build kcmp selftest"
-endif
+all: kcmp_test
 
 run_tests: all
 	@./kcmp_test || echo "kcmp_test: [FAIL]"